In movable manufacturing machines, e.g., an industrial robot, etc., troubles due to contact with an external fixed body cannot be avoided by any means. It is the present stage that an approach is employed to detect such an unfavorable contact to stop the operation of the robot as soon as possible, thus to prevent the robot and the work from being damaged.
The appearance of a typical industrial robot using a weight body as a work is shown in FIG. 3 as the side elevational view.
As well known, this industrial robot designated by reference numeral 20 is provided with an arm 21, a swivel portion 22 connected to the arm 21, a gripper portion 24 connected to the swivel portion 22 through a contact detector 23 for movable object, and a spot gun 26 for work affixed to the gripper portion 24. Thus, this industrial robot carries out a welding work by making use of chips 27 provided on the spot gun 26.
FIGS. 4A-4C show a contact detector for a movable object, designated by reference numeral 23, which has been already proposed by the applicant (Japanese Utility Model Application Laid Open No. 61492/1987). A member 1 is affixed to a wrist flange of the robot by means of bolts 15. A member 2 is connected to the member 1 by means of bolts 14. These members 1 and 2 are affixed to the robot side. A member 3 for fitting a work is connected to the member 2 side. The attachment of the member 3 is carried out by assembling a spherical bearing 4 into the member 3, inserting a shaft 5 into the spherical bearing 4, thereafter inserting a resilient body 6 comprised of a coned disk spring into the portion between the shaft 5 and the member 2, and screw-connecting a nut 7 over the front end portion of the shaft 5. Bushes 8 are assembled into the member 2 on the robot side and the member 3 on the work side, respectively. Steel balls 9 are put between these bushes 8. In this instance, these steel balls 9 are held by a retainer 10 and put between the bushes 8. Attachment thereof is carried out by tightening or fastening the nut 7. Reference numeral 11 denotes a switch affixed to the member 1. This switch 11 is inserted into the shaft 5 and is set so that it is turned ON in response to detection of a displacement of the shaft 5. In this example, a bolt 12 for adjustment of operating distance is opposite to the front end surface of the switch 11. In this figure, reference numeral 11a is a nut for fixing the switch 11 serving as a double nut, reference numeral 11b is a lead wire for the switch 11, reference numeral 11c is a hole for penetrating the lead wire 11b therethrough, and reference numeral 12a is a coned disk spring for preventing loosening of the bolt 12 for adjustment.
The operation of the arrangement stated above will be described.
When a moment M.sub.1 in a direction indicated by an arrow is applied to the member 3, the member 3 raises the other shoulder thereof against the coned disk spring 6 using a steel ball 9 as a point of support. Thus, the shaft 5 is concurrently subjected to displacement. As a result, the contact of the switch 11 is opened, whereby an ON signal is output.
When a torsional external force M.sub.2 is applied, a steel ball 9 slips out of bushes 8 provided in the members 2. Similarly, the shaft 5 is subjected to displacement in a direction of opening the switch 11. Thus, an ON signal is output from the switch 11.
When tensile forces f and F are applied, an operation similar to the above is performed. Namely, the member 3 raises the other shoulder thereof against the coned disk spring 6 using a steel ball 9 as a point of support, thus allowing the shaft 5 to be subjected to displacement in a direction of opening the switch 11. As a result, an ON signal is output.
As just described above, in accordance with this detector, when an external force is applied to the work side member 3, a signal indicative of contact with an external fixed body is output from the switch 11. By employing a device to stop a robot using this signal, damages due to contact trouble of robot can be prevented.
This contact detector is of a structure such that it is automatically returned to a normal state when a contact object is removed. In addition, since reproducibility is ensured, restration can be made in a short time.
However, with such a conventional detector, when an external force is exerted on steel balls 9 between bushes 8, there is the possibility that the contact surface of the bush 8 may be deformed because of the hardness of the bush 8 is low. FIGS. 5A and 5B show the case that a torsional external force M.sub.2 is exerted on the member 3. A pressing force P by the resilient body 6 is exerted on the steel ball 9 (FIG. 5A). When an external force M.sub.2 is exerted in this state, the steel ball 9 attempts to ride on the shoulder portion of the bush 8 (bush on the lift side in the figure) of the robot side member 2 to firmly come into contact therewith. By this contact, there is produced a deformation C such that the contact surface of the bush 8 becomes depressed (FIG. 5B). Since detection accuracy initially set varies by such a deformation C, it is impossible to permanently maintain the detection accuracy. Furthermore, steel balls 9 are simply put between bushes 8 and held therebetween. For this reason, a clearance is apt to occur between steel balls 9 and bushes 8. The positioning accuracy at the working front end of the robot is lowered by this clearance.